When a three-piece combination oil ring including a pair of upper and lower side rails and a spacer expander arranged therebetween is used, sludge such as unburned carbon or carbon produced from a product of combustion of lubricant oil tends to be deposited between the spacer expander and the side rails. With a progression of deposition, the side rails and the spacer expander stick to each other due to deposits, and the side rails cannot follow the shape of a cylinder bore, so that the oil ring may not be able to exert predetermined performance as an oil ring.
Meanwhile, in Patent Literature 1, a hole large enough to cause sludge to pass therethrough is formed in a center portion between convex and concave portions of an expander each having a substantially flat surface.
Also, in Patent Literature 2, a groove is formed in a surface that is close to a side rail of an upper piece and a surface that is close to a side rail of a lower piece of a spacer expander, and the groove communicates with a through hole formed in a flange portion for pressing the side rails in a combination oil ring. Also, since the groove extends radially and is open on an outer circumferential side, oil flows in from the opening portion to form a flow of oil from the outer circumferential side to an inner circumferential side, so that the oil containing sludge is easily discharged from the through hole in the flange portion, thereby making it difficult for deposits to accumulate.